Method for soldering miniaturised components to a base plate

ABSTRACT

The invention relates to a method for fixing a miniaturized component, especially comprising a micro-optical element, to a pre-determined fixing section of a baseplate by means of a soldered joint. Said baseplate comprises an upper side and a lower side, and the component comprises a base surface. At least the fixing section of the baseplate is coated on the upper side by a metallic layer which is applied at least in the fixing section in a continuously plane manner and thus without interruptions. Solder material is applied at least to the fixing section coated with the metallic layer. In one step of the method, the component is arranged above the fixing section, the base surface of the component being positioned above the solder material in such a way that they do not touch, are vertically interspaced and form an intermediate region between each other. In another step, thermal energy is supplied, especially by means of a laser beam, in a region which is locally limited essentially to the fixing section, in order to melt the solder from the lower side of the baseplate, such that the intermediate region is filled by drop formation of the melted solder, fixing both sides.

The present invention relates to a. method for highly precise fixing ofminiaturized components to a baseplate by means of a solder joint. Inparticular, the invention relates both to a laser soldering method forfixing microoptical components to a baseplate which is at least partlytransparent to laser beams and is coated with a metallic layer and tothe baseplate obtained by this method and a substrate for use in themethod.

Different methods for fixing miniaturized components by means of asolder joint are known from the prior art.

WO 99/26754 describes a method for soldering miniaturized components toa baseplate. The baseplate is at least partly coated with a lattice-likemetal pattern or metal structure. According to WO 99/26754, thepatterned structure can be formed by a lattice of planar elements or anumber of stripes which preferably intersect one another perpendicularlyand form a lattice or some other structure which is characterized by auniform alternation of metal regions and free regions. The planarelements may assume a rectangular shape, a square shape, a round shapeor any shape which is suitable for use in this method. Here, the patternstep width should be at least one order of magnitude smaller than thedimensions of the component side to be fixed. The solder material ispreferably applied to the component side to be fixed or optionally tothe metal pattern of the baseplate. The component is arranged above thebaseplate, the metal pattern and the solder material layer of thecomponent or the solder material layer on the metal pattern and thecomponent side being located opposite one another without contact and avertical distance apart. Heat energy is then supplied from the uncoatedside of the baseplate for melting the solder material or the soldermaterial layer on the coated side on which the component is alsoarranged until drop formation of the solder material occurs, with theresult that the solder material drops fill the intermediate spacebetween the component and the baseplate for mutual fixing. Themetallized sample regions provide a fixing region for the soldermaterial, while the metal-free regions make it possible for a sufficientquantity of energy to pass through the baseplate in order to melt thesolder material. When heat energy is supplied, a part of the energy thuspasses through the uncoated regions of the metal pattern. The part whichreaches those regions of the metal pattern which are coated with themetal either heats up the metal layer or is reflected. The energy ispreferably applied by means of a laser beam. Owing to the latticestructure of the metal pattern on the baseplate, the choice of asuitable light absorption coefficient of the metal layer is particularlyproblematic since, on the one hand, the baseplate is not permitted tooverheat and, on the other hand, a certain minimum temperature isrequired for carrying out the process. Usually, metal of the metalpattern covers about 70% of the surface of the baseplate. Less than 30%of the power of the laser beam is used for heating the solder materialsince one part of the approximately 70% of the remaining power heats upthe baseplate and the other part is reflected.

For carrying out the soldering process, more than 15 W are required forthe duration of 2 seconds. Different types of metal coatings arerequired depending on the chosen material of the baseplate. Moreover, adiscrete effect occurs owing to the metal pattern if the solder materialwets only those regions of the metal pattern which are coated with themetal layer but—in particular owing to the flow behaviour and thesurface tension—not the uncoated metal-free regions of the metalpattern. Particularly in the case of a metal pattern which is coarse inrelation to the side of the component side to be fixed, the result maytherefore be asymmetric off centre soldered seams which—owing to theshrinkage of the solder material which inevitably takes place duringsolidification—are associated with a change in position of the componentand/or a skew position.

In practice, it has been found that the solder material coolssubstantially more rapidly close to the transverse sides of thecomponent than the remainder of the solder material. This can lead toasymmetries of the solidified solder material.

Since the fixing is effected by soldering of two different materials,different coefficients of thermal expansion in the case of the soldermaterial and of the baseplate cause high stresses and distortions afterthe soldering process has been carried out. Depending on the mechanicalresistance of the carrier material, parasitic local deformation mayoccur in the region of the solder joint owing to the high stresses.

By means of the known method described above, it is possible to achieveaccuracies of about 2 microns in the 6 degrees of freedom. Particularlyin the mounting of components which contain or carry microopticalelements, however, even higher accuracies of mounting are required.

There is therefore the need to improve the method described above anddisclosed in WO 99/26754 in order to achieve a higher accuracy ofmounting of miniaturized components, in particular microopticalelements, on a carrier plate and to optimize the entire method from theeconomic point of view.

Furthermore, flux-free and/or lead-free soldering is scarcely possiblein the method described above. Experiments have shown that the use offlux-free solder material comprising Sn96Ag4 and a metal layer coatedwith tin, nickel and gold does not lead to the desired satisfactoryresults if the fixing is effected on a metal pattern as described in WO99/26754 since a metal pattern has insufficient wettability.

Numerous further methods for fixing small components on a carrier plateare known from the area of the equipping technology for electronicsurface-mounted circuits, SMD technology. In the case of electroniccircuits, however, the requirements are completely different from thosein the area relating to the mounting of microoptical components sincelower precisions are required in the case of electronic circuits andhighly accurate orientation in all 6 degrees of freedom is not required.Moreover, electronic circuits must by definition be composed ofinterrupted metal layers since it would be inexpedient to mount a largenumber of electronic components on a circuit board having a singleuninterrupted metal layer. The completely free positionability ofcomponents on a carrier plate is neither required nor desired in thecase of electronic circuits but is of decisive importance, for example,for building up a microoptical system on a carrier plate. Thus,approaches from electronic circuit board equipping technology aresuitable only to a limited extent.

It is an object of the present invention to provide a method for fixinga miniaturized component, in particular comprising at least onemicrooptical element, to a baseplate by means of a solder joint, whichmethod is distinguished by increased positional accuracy of thecomponents compared with the known method and high cost-efficiency.

This object is achieved by realizing the features of the independentClaims. Features which develop the invention in an alternative oradvantageous manner are evident from the dependent Patent Claims.

According to the invention, in order to fix a miniaturized component,comprising in particular a microoptical element, with its base surfaceto a baseplate by means of a solder joint, the baseplate is coated witha metal layer, the metal layer being applied in a continuously planarmanner and hence being free of interruptions. The baseplate and themetal layer form a so-called substrate. The solder material is appliedto the metal layer of the baseplate. Thereafter, the component isarranged above the baseplate, the solder material and the base surfaceof the component being present opposite one another without contact, avertical distance apart and so as to form an intermediate space. Bysupplying heat energy in a region of the bottom of the baseplate, whichregion is locally limited substantially to the fixing section, thesolder material is melted so that the intermediate space between themetal layer and the base surface of the component is filled by dropformation of the molten solder material for mutual fixing—optionallyassisted by lowering of the component. As a result of solidification ofthe solder material, the mutual fixing is implemented.

Owing to the continuous, extensive metal layer, a substantially higherfinal accuracy of positioning of the component on the baseplate isachieved in comparison with the method of WO 99/26754 which employs themetal pattern, since the continuous metal surface is wet easily and morehomogeneously with the solder material than is the case with aninterrupted metal structure.

Moreover, the higher absorption coefficient of the metal layer resultsin a reduction of 40% in the laser beam power required for soldering,compared with the method of WO 99/26754. Thus, a laser power of lessthan 10 W for 2 seconds is sufficient for carrying out the soldering. Byreducing the required power, the danger of damage to the components tobe fixed also decreases considerably, especially if they aremicrooptical components which are very heat-sensitive. The laser beam issubstantially better absorbed by the metal layer than in the knownmethod. It is therefore possible to use alloys having a high meltingpoint. For the method according to the invention, the absorptioncoefficient of the metal layer need only be maximized to permitsoldering on different types of baseplates, whereas the soldering powerof the laser need only be adapted to the thermal expansion of thebaseplate.

Since, in the method of the present invention, an uninterrupted,continuously planar metal layer is used, a considerable cost reductionin the production of the substrate is possible since a continuous metallayer can be substantially more easily produced than a metal pattern.

Of course, it is not necessary for the continuously planar metal layerto be applied to the entire baseplate. It is of course possible to applyto the baseplate a plurality of cohesive metal layers which are notconnected to one another. In that fixing section in which the componentis to be fixed with its base surface to the baseplate, however, themetal layer is substantially free of interruptions and continuouslyplanar. It is possible to apply a plurality of planar elements to abaseplate in a pattern, which elements are each cohesive andcontinuously planar, but each planar element represents a separatefixing section to which the component is to be fixed, so that theprojection of the base surface of the component is completely covered byone planar element. Continuously planar also does not inevitably mean ametal layer having an absolutely uniform layer thickness but asubstantially cohesive surface which has substantially no interruptions,apart from small interruptions which occur in particular in the case ofsmall layer thicknesses but are not produced artificially in the form ofa patterned structure.

In an embodiment of the invention, the solder material is placed on themetal layer in the form of a flat piece which has, for example, theshape of a truncated cylinder. Thereafter, the component is arrangedwith its base surface above the solder material. An energy source, forexample laser, is aligned below the baseplate and emits an energy beamfrom the bottom of the baseplate. The energy beam passes through thebaseplate and strikes the metal layer on the top of the baseplate. Themetal layer is subjected to localized heating here and thus heats thesolder material, in a manner comparable with a stove plate. The flatpiece of solder material melts and, in the liquefied state, assumes adrop-like form owing to the surface tension of the solder. Moreover, thesolder material expands by a few hundred microns owing to the heating.As a result of the drop form and the expansion, the distance between thesolder material and the base surface of the component decreases. Wettingof the base surface of the component with solder material thus occurs,so that a solder joint can be produced.

Alternatively, it is possible to reduce the intermediate space betweenthe metal layer and the base surface of the component by lowering thecomponent from a starting position towards the baseplate so that thebase surface of the component dips into the molten solder material andthe base surface of the component is thus wet with the solder material.Optionally, the component is withdrawn again to the starting positionthereafter—before the beginning of solidification. In this embodiment,but also in the first one, it is conceivable to apply the soldermaterial also or exclusively to the base surface of the component.Since, however, heating of a solder material which is bonded to themetal layer directly heated by the radiation is more effective, betterresults are obtained with a solder material applied directly to themetal layer.

In a further embodiment of the invention, the solder material is appliedat least to a part of the metal layer of the baseplate, discretely inthe form of a multiplicity of solder material elements a distance apart,for example in the form of a spot pattern, or in a continuously planarmanner in the form of an uninterrupted layer. Here, this part issubstantially larger than the actual fixing section which an individualcomponent occupies. Optionally, the entire metal layer is coated withthe solder material. An energy source, for example a laser, is aligned,as in the above method, below the baseplate and emits an energy beamfrom the bottom of the baseplate. The energy beam passes through thebaseplate and strikes the metal layer. The latter heats up in a narrowlylocalized region and melts the solder material layer or the soldermaterial pattern within this region. Drop formation occurs in thisregion owing to the surface tension of the solder. Thus, optionally byadditional lowering of the component, the base surface of the componentis wet with solder material so that a solder joint can be produced.

In another embodiment of the invention, a solder material in the form ofa flat piece, in particular a truncated cylinder, is arranged on thebaseplate, the cross-section of the flat piece being smaller than thecross-section of the base surface of the component, so that theprojection of the base surface of the component onto the metal layer ofthe baseplate completely covers the cross-section of the flatpiece—prior to melting. By reducing the cross-section of the flat piece,even higher accuracy of fixing is achievable.

By means of the soldering method according to the invention, accuraciesof fixing in the region of 0.25 micron are achieved, so that this methodis outstandingly suitable for the highly precise mounting ofmicrooptical components on a baseplate.

The method is described in detail below. The invention is illustratedbelow by a specific embodiment with reference to figures.

The base surface of the miniaturized component must have goodwettability for solder material. This base surface may be flat orconvex, for example in the form of a convex spherical surface section orof a convex cylinder lateral surface section. A spherical base surfacesimplifies exact alignment of the component owing to the symmetry. Bymeans of a rotationally symmetrical solder joint between the baseplateand the component, a stable joint having little distortion in the caseof shrinkage and having good repeatability is achieved. This increasesthe process reliability and is particularly important for massproduction.

Since the energy supply is from that side of the baseplate which isopposite the side coated with the metal layer, and hence the energy forheating the metal layer is supplied through the baseplate, it isnecessary to choose a baseplate which is substantially transparent forthe wavelength of the energy provided. If a laser beam is used as theenergy source, the baseplate should have high transparency for the laserbeam wavelength.

Moreover, the coefficients of thermal expansion of the baseplate and ofthe metal layer must correspond to the extent that no tearing orbuckling of the metal layer should occur during or after the supply ofthe energy. Ideally, the coefficient of thermal expansion of thebaseplate is equal to that of the metal layer. The material used for theproduction of the baseplate should be capable of withstanding highthermal loads since, on supplying energy, for example by means of alaser or UV, a part of the energy passed through the baseplate isinevitably absorbed by the baseplate. Moreover, strong heating occurs ina region of the metal layer which has high conductivity, and it is forthis reason that the baseplate too is strongly heated in a locallylimited region. If the baseplate comprises a material having poorthermal conductivity, for example glass, ceramic or glass ceramic, thereis a high energy concentration in the locally limited region. This couldlead to failure of the material in the case of an unsuitable choice ofmaterial. If the mutual alignment of a plurality of components isimportant, it is also necessary to ensure that no material which has ahigh coefficient of thermal expansion is chosen as the baseplate sincethe alignment of the optical components changes in the event of heatingof the baseplate. This would inevitably lead to optical errors in theoptical system. Suitable materials for the baseplate are, for example,glass, sapphire, ceramic, glass ceramic, silicon or Pyrex. Furthersuitable materials are known from the prior art.

The metal layer may be formed from a plurality of layers of differentmetals and alloys and optionally coated with an antioxidant, flux, etc.Preferably, the alloy of the metal layer should have little tendency tooxidize and should contain gold.

In an embodiment of the invention, a sapphire piece or a Pyrex waferhaving a thickness of about 1 to 2 millimetres is chosen as thebaseplate. The wafer is coated with a metal layer comprising chromium,nickel and gold. The metal layer has a thickness of about 1 micron. Thesolder material chosen is, however, SnPb or Sn96Ag4, which fills a gapbetween the metal layer and the base surface of the component of 0.2 to0.5 millimetre.

The shrinkage of the solder material during cooling inevitably ensures avertical offset of the component perpendicularly to the baseplate. Ithas good repeatability and is a function of the gap between thebaseplate and the base surface of the component. For compensation ofthis vertical shrinkage, it is possible to position the componentcorrespondingly higher and to take the shrinkage into account in theprepositioning.

In a further embodiment of the invention, the accuracy of mounting isfurther increased, especially in the case of an inclined component, byreducing the cross-section of the solder material, for example thediameter of the flat piece, so that this cross-section is smaller thanthat of the base surface of the component. In this case, the diameter dof the flat piece of solder material is smaller than the diameter D ofthe base surface of the component. At the latest after solidification ofthe solder material, the diameter of the solidified solder material issmaller than the diameter D of the base surface. This results in lessasymmetries on solidification of the solder material, which may occur inparticular because of the fact that the solder material coolssubstantially more rapidly on the transverse surfaces than in themiddle. Especially in the case of inclined mounting of a component, itis therefore advantageous to provide a solder joint with little soldermaterial since, in this case, shrinkages do not have an effect to suchan extent. Moreover, there are in this case fewer stresses which resultfrom the cooling and the associated shrinkage, so that the manufacturingaccuracy is further increased.

The method is suitable in particular for use in an automatic, flux-freelaser soldering process since substantially all required steps can becarried out by handling robots which have a highly accurate positionsensor system. Thus, each component can be positioned in a highlyprecise manner freely in space in all 6 degrees of freedom. The freepositionability, which is not inevitably limited by predeterminedregions on the baseplate, is a further advantage of the invention. Ifthe entire baseplate is coated with the metal layer, it is possible toposition the component in any desired position on the baseplate eitherby free positioning of a flat piece of solder material or by means of asolder material layer.

This technique described here is suitable in particular for highlyaccurate fixing of microoptical components, oriented in all 6 degrees offreedom. Thus, the miniaturized component may serve as a holder for amicrooptical element, for example a lens, an optical fibre, a laserdiode, etc. Such microoptical components typically have a diameter ofthe order of magnitude of 2.6 mm and a height of 3.5 mm. A possibleexample of a holding device for a microoptical component is described inEP 1127287 B1.

A further embodiment of the invention comprises a baseplate having aplurality of miniaturized components which are arranged within acomponent region of the baseplate and in each case have at least onemicrooptical element. At least the component region of the baseplate iscoated on the top with at least one metal layer, the metal layer on thetop of the baseplate being applied in a continuously planar manner atleast in the component region and hence being free of interruptions. Thecomponents are fixed with the base surface above in each case one solderjoint on the metal layer. The baseplate is transparent to laserradiation. The component region is a part of the baseplate on which aplurality of parts are arranged on the cohesive metal layer. In apossible embodiment, the metal layer is applied in a continuously planarmanner to the entire top of the baseplate and hence substantiallywithout interruptions. In this case, the component region is formed bythe entire baseplate. The solder material of the solder joint preferablyhas a concave outer surface.

In a special embodiment, at least one of the plurality of componentscomprises a holder for holding a supporting part on which the at leastone microoptical element in each case is fixed, the holder beingconnected to the supporting part, and the supporting part to themicrooptical element, by means of soldering points. Such a holder isalso described in EP 1127287 B1.

The substrate for use in the method according to the invention is formedby a baseplate which is transparent to laser radiation and is coated onone side with at least one metal layer which is applied in acontinuously planar manner substantially to the entire side and is thussubstantially free of interruptions. Optionally, the substrate is coatedwith a layer of solder material which is applied in a continuouslyplanar manner at least to a part of the metal layer of the baseplate, sothat the layer of solder material is free of interruptions in the part.Alternatively, the layer of solder material is applied, in at least onepart on the metal layer of the baseplate, in a pattern comprising amultiplicity of solder material elements a distance apart.

The invention is described in more detail below with reference tospecific embodiments shown schematically in the drawings. Specifically:

FIG. 1 a, 1 b show the arrangement of a component, of a solder materialin the form of a small piece and of a substrate before (FIG. 1 a) andafter (FIG. 1 b) the production of the solder joint;

FIG. 2 a, 2 b show the arrangement of a component, of a solder materialin the form of a large flat piece and of a substrate before (FIG. 2 a)and after (FIG. 2 b) the production of the solder joint;

FIG. 3 shows the determination of the diameter d of a solder material inthe form of a flat piece in the case of component inclination ofα_(max)=±5°;

FIG. 4 a, 4 b show a component and a substrate with a layer of soldermaterial which is applied in a pattern comprising a multiplicity ofsolder material elements a distance apart, before (FIG. 4 a) and after(FIG. 4 b) the production of the solder joint;

FIG. 5 a, 5 b show a component and a substrate with a layer of soldermaterial applied in a continuously planar manner, before (FIG. 5 a) andafter (FIG. 5 b) the production of the solder joint; and

FIG. 6 shows a component which comprises a holder for holding asupporting part on which a microoptical element is fixed.

FIG. 1 a and FIG. 2 a each show a baseplate 1 having a top 8 and abottom 9 in a state before production of a solder joint. The baseplate 1which is transparent to laser radiation is coated on its top 8 with ametal layer 5 which is applied in a continuously planar manner so thatit is substantially free of interruptions. The baseplate 1 and the metallayer 5 form a so-called substrate. Within a fixing section 7 on themetal layer 5 a solder material 6 a, 6 b in the form of a flat piece,which, in the not yet molten state, has the shape of a truncatedcylinder but may also have another shape is applied to the metal layer5. The fixing section 7 is that section on which a single component 2 isto be fixed or is fixed. A miniaturized component 2 which carries amicrooptical element 3 is arranged above the fixing section 7 of thebaseplate 1 so that the solder material 6 a, 6 b and a convex basesurface 4 of the component 2 are present opposite one another withoutcontact, a vertical distance apart and so as to form a verticalintermediate space. The component 2 is positioned and held in a highlyprecise manner by means of a robot station (not shown), the expectedvertical shrinkage being taken into account. In FIG. 1 a, the soldermaterial 6 a in the form of a flat piece has a diameter d1 which issmaller than the diameter D of the base surface 4 of the component 2, sothat d1<D, whereas the solder material 6 b in the nonmolten state inFIG. 2 a has a diameter d2 which is equal to the diameter D, so thatd2=D.

By supplying laser radiation locally limited to the fixing section 7 andpassing through the baseplate 1, in the form of a laser beam 11, fromthe bottom 9 of the baseplate 1, the metal layer 5 is strongly heated atleast within the fixing section 7 and acts as a sort of stove plate, sothat the solder material 6 a, 6 b melts, forms a drop owing to thesurface tension, wets the base surface 4, flows into the intermediatespace and thus produces a joint between the metal layer 5 and the basesurface 4. Thereafter, the laser radiation 11 is deactivated again andtime is allowed for the molten solder material 6 a′, 6 b′ to solidify.FIGS. 1 b and 2 b show the state after production of the joint with themolten solder material 6 a′, 6 b′. In both cases, the vertical distancebetween the component 2 and the baseplate 1 decreases as a result of theshrinkage of the solder material 6 a′, 6 b′. Moreover, the diameter ofthe molten solder material 6 a′, 6 b′ decreases in comparison with thediameter d1, d2 of the solder material 6 a, 6 b in the nonmolten state,it being true both in FIG. 1 a for d1<D and in FIG. 2 a for d2=D thatthe diameter of the molten solder material 6 a′, 6 b′ is smaller than D.Here, the molten material 6 a′ in FIG. 1 b has a smaller diameter thanthe molten solder material 6 b′ in FIG. 2 b. The lateral surface of thesolder material 6 a′, 6 b′ has a concave shape. FIG. 3 shows thedetermination of the diameter d of a solder material 6 a in the form ofa small flat piece in the case of an inclination of the component 2 ofα_(max)=±5°. Assuming that the diameter D of the base surface 4 of thecomponent 2 is, for example, D=2.6 mm, the radius of the round part ofthe base surface 4 is r=1.6 mm and the angle of inclination of thecomponent 2 is α_(max)=±5°, a maximum diameter of d_(max)=2.43 mm isobtained for the diameter d of the contacting solder material 6 a in theform of a flat piece. The ideal distance arises from the requirementthat the solder material 6 a should not be in contact with the basesurface 4 before production of the joint, even in the case of aninclined component 2. In order to meet this requirement, the diameter dof the small flat piece of solder material 6 a is reduced by 20%relative to d_(max), so that d=2.43·(1−0.2)=1.94 mm.

FIGS. 4 a and 4 b show a component 2 and a substrate comprisingbaseplate 1 and metal layer 5 with a layer of solder material 6 capplied in a pattern comprising a multiplicity of solder materialelements 6 c″ a distance apart, before (FIG. 4 a) and after (FIG. 4 b)the production of the solder joint. The solder material here 6 c in theform of a pattern is applied to the entire metal layer 5. A locallylimited laser beam 11 melts a plurality of solder material elements 6 c″of the solder material layer 6 c and causes them to coalesce to form adrop of solder material 6 c′ which, as in FIGS. 1 a/1 b and 2 a/2 b,produces the joint between the base surface 4 of the component 2 and themetal layer 5 of the baseplate 1. FIGS. 5 a and 5 b, on the other hand,show a component 2 and a substrate having a layer of solder material 6 dapplied in a continuously planar manner, before (FIG. 5 a) and after(FIG. 5 b) the production of the solder joint. Here, a laser beam 11melts a part of the solder material 6 d, which forms into a drop ofsolder material 6 d′ and produces a joint between the component 2 andthe baseplate 1.

FIG. 6 shows an embodiment of a component 2 which comprises a holder 2′for holding a supporting part 2″ on which a microoptical element 3 isfixed, the holder 2′ being joined to the supporting part 2″, and thesupporting part 2″ to the microoptical element 3 by means of solderingpoints 10. In order to permit a solder joint 10, the component 2 has abase surface 4.

1. Method for highly precise, freely positionable fixing of a miniaturized component, in particular comprising a microoptical element, to a predetermined fixing section of a baseplate by means of a solder joint, the baseplate having a top and a bottom and the component having a base surface, at least the fixing section of the baseplate being coated on the top with at least one metal layer, and solder material being at least partly applied at least to the fixing section of the baseplate, which section is coated with the metal layer, comprising the steps highly precisely arranging the component above the fixing section of the baseplate, the solder material and the base surface of the component being present opposite one another without contact, a vertical distance apart so as to form an intermediate space, supplying heat energy in a region locally limited substantially to the fixing section for at least partial melting of the solder material from the bottom of the baseplate so that, as a result of drop formation of the molten solder material, the intermediate space is filled for mutual fixing, and waiting for the mutual fixing, the metal layer being applied to the top of the baseplate at least in the fixing section in a continuously planar manner and thus being without interruptions in the fixing section.
 2. Method according to claim 1, the metal layer being applied in a continuously planar manner substantially to the entire top of the baseplate and thus being without interruptions substantially over the entire top of the baseplate.
 3. Method according to claim 1, the step of supplying heat energy comprising the following part-steps: supplying heat energy in a region locally limited substantially to the fixing section for melting the solder material from the bottom of the baseplate and reducing the intermediate space between the molten solder material and the base surface of the component by lowering the component from a starting position towards the baseplate so that the base surface of the component dips into the molten solder material and the base surface of the component is wet.
 4. Method according to claim 3, comprising the further part-step: repositioning of the component in the starting position.
 5. Method according to claim 1, the supply of heat energy being effected from the bottom of the baseplate by a beam of electromagnetic waves which is directed at the fixing section, heats the metal layer and thus melts the solder material.
 6. Method according to claim 5, the beam being a laser beam and the baseplate being transparent to laser radiation.
 7. Method according to claim 1, the base surface of the component being convex.
 8. Method according to claim 7, the base surface of the component having the form of a convex spherical surface section.
 9. Method according to claim 7, the base surface of the component having the form of a convex cylinder lateral surface section.
 10. Method according to claim 1, the solder material being applied to at least that fixing section of the baseplate which is coated with the metal layer, in a manner such that, in the still nonmolten state of the solder material, the base surface of the component which is projected onto the top of the baseplate completely covers the cross-sectional area of the solder material.
 11. Method according to claim 1, the solder material being applied at least to that fixing section of the baseplate which is coated with the metal layer, in a manner such that, in the still nonmolten state, it has the form of a flat piece, in particular that of a flat truncated cylinder.
 12. Method according to claim 1, the solder material in a region containing the fixing section being applied to the metal layer, the region being substantially larger than the fixing section contained therein.
 13. Method according to claim 12, the solder material being applied in a continuously planar manner in the region on the metal layer of the baseplate and thus being free of interruptions in the region.
 14. Method according to claim 12, the solder material being applied in the region on the metal layer of the baseplate in a pattern comprising a multiplicity of solder material elements a distance apart.
 15. Baseplate comprising a plurality of miniaturized components which are arranged highly precisely and freely positioned within a component region of the baseplate and in each case comprise at least one microoptical element, the baseplate having a top and the components each having a base surface, at least the component region of the baseplate being coated on the top with at least one metal layer, the components being fixed with the base surface by means of a solder joint in each case to the metal layer, and the baseplate being transparent to laser radiation, characterized in that the metal layer is applied in a continuously planar manner to the top of the baseplate at least in the component region and is thus free of interruptions in the component region.
 16. Baseplate according to claim 15, characterized in that the metal layer is applied in a continuously planar manner substantially to the entire top of the baseplate and is thus free of interruptions substantially over the entire top of the baseplate.
 17. Baseplate according to claim 15, characterized in that the solder material of the solder joint has a concave outer surface.
 18. Baseplate according to claim 15, characterized in that the base surface of the component is convex.
 19. Baseplate according to claim 15, characterized in that at least one of the plurality of components comprises a holder for holding a supporting part on which the at least one microoptical element in each case is fixed, the holder being joined to the supporting part, and the supporting part to the microoptical element, by means of soldering points.
 20. Substrate for use in the method according to claim 1, characterized by a baseplate transparent to laser radiation, at least one metal layer and a layer of solder material, the baseplate being coated on one side with the at least one metal layer, the metal layer being applied in a continuously planar manner substantially to the entire side of the baseplate and thus being free of interruptions substantially over the entire side of the baseplate and the layer of solder material being applied at least to a part of the metal layer of the baseplate in a pattern comprising a multiplicity of solder material elements a distance apart.
 21. Substrate according to claim 20, characterized in that the layer of solder material is applied to the entire metal layer. 